1. Field of the Invention
This invention relates to techniques for determining properties of formations surrounding an earth borehole and, more particularly, to a technique for downhole determination of the resistivity of fluids extracted from earth formations.
2. Background of the Invention
The resistivity of fluids in earth formations is known to be a very important indicator of the nature of the formations. The well logging art contains numerous approaches to indirect and direct determination of formation fluid resistivity. [In the present application, any references to determination or use of resistivity are intended to generically mean conductivity as well, and vice versa. These quantities are reciprocals, and mention of one or the other herein is for convenience of description, and not intended in a limiting sense]. Several techniques are commonly used to measure the resistivity of the formations (the fluid and the matrix containing the fluid) using measurements from a logging device in the borehole, which, for example, can employ electrodes to perform so-called “normal” or “lateral” resistivity logging, or coils to perform induction logging. Electrodes can also be used for spontaneous potential (“SP”) logging to determine formation water resistivity. It has also been proposed that formation resistivity can be determined by measuring the impedance looking into an antenna coil in a borehole, the antenna also serving a source of RF energy for a companion NMR technique (see U.S. Pat. No. 5,831,433).
The resistivity of formation fluid samples drawn from the formations by a logging device, such as a so-called formation testing tool, can be measured directly downhole, and this is typically performed using electrodes spaced apart in the fluid. However, a problem with contact measurements of fluid resistivities is that the electrodes can be wetted by one of the fluids. This may bias the measurements. Another problem is that when measurements are made in vertical or highly deviated wells, the effects of gravity segregation of the fluids typically results in measurement errors unless the fluids are flowing.
It is among the objects of the present invention to provide a method for downhole determination of formation fluid resistivity which is efficient to practice and which overcomes shortcomings of prior art approaches.